When a level of a rectangular signal changes, noises will arise due to harmonic components included in the sharp rising portions and the sharp falling portions of the rectangular signal. For instance, if the rectangular signal having the frequently changing levels is used in vehicle-mounted electronic devices, radio noises are generated. It is effective to use a trapezoid signal in place of the rectangular signal for reducing such noises.
JP-A-52-112263 discloses a waveform shaping circuit, which changes the inclination of one of the rising slope and the falling slope of a trapezoid signal in the middle of the slope. This circuit increases the inclination of the sharper one of the inclination-changed slopes to be sharper than the inclination of the other slope, which is not inclination-changed. This circuit decreases the inclination of the less sharp one of the inclination-changed slopes to be less sharp than the inclination of the other slope, which is not inclination-changed. Specifically, this circuit has a capacitor, and changes a charging and discharging current of the capacitor in steps according to the terminal voltage of the capacitor.
FIG. 6 shows an example of a trapezoid signal generating circuit, and FIG. 7 shows the waveforms of operation signals of the trapezoid signal generating circuit 1. When an input signal Sin becomes a high level H, a switch 6 turns off via an inverter 5. A capacitor 2 is charged with an output current I1 of a constant current circuit 3. Thus, the output voltage Vo outputted via a buffer 7 linearly increases to a fixed level. When the input signal Sin becomes a low level L, the switch 6 turns on. The capacitor 2 is discharged with a current I1 which corresponds to the difference between the output currents of the constant current circuits 3 and 4. Thus, the output voltage Vo linearly decreases to zero.
In the voltage Vo (trapezoid signal) thus generated, the inclination changes in a stepwise manner at the shoulder portions of its rising portion and falling portion (increase/decrease starting portion or increase/decrease ending portion). As a result, it is difficult to sufficiently reduce the harmonic components. Therefore, the above prior art proposes to reduce changes of the inclination at the shoulder portions by changing the charging and discharging current in steps. In case of changing the charging and discharging current in steps in accordance with the terminal voltage of the capacitor 2, however, comparators in the number corresponding to the number of change steps are necessitated. The circuit size will be complicated more as the waveform is smoothed more.